Photo interpolation is the
process by which the number of pixels comprising an image is increased to allow
printing enlargements that are of higher quality than photos that are not
interpolated. Interpolation is commonly needed to make quality large
prints from digital photos and film-scanned images.

Interpolation is the
process by which a small image is made larger. Software tools stretch the
size of the image and generate
pixels to fill in the blanks. Interpolated images
produce smoother lines and a better large print than if the original, small
image was simply printed large.

Prints significantly
larger than 8.5x6.4 would likely benefit from interpolation. It really
depends on the personal taste of the viewer. An 8x10 image size would be
roughly 240dpi which would probably look just fine.

200dpi is often used for
prints that will be viewed from a distance.

How does a 5mp (2560x1920) digital image compare with 35mm film?

A drum scanned 35mm slide at 4000 dpi will yield an image that approximately
4000x6000. Most pro labs that print from slides will scan the slides and print
the image file. At 4000dpi, the result is a 24 megapixel file. Quite a bit
more than the 5mp Olympus c-5050z but includes an additional degree of degradation
involved with scanning.

What degree of
interpolation is necessary for 10x15 and 20x30 prints given the 5mp example?

If using a resolution of 300dpi and making a 10x15 print, a 3000x4500 would be
needed. For a 20x30 image, 6000x9000 would be needed.

Less dpi is required for
larger prints since they are usually viewed from further away... let's say
200dpi. So, for a 20x30 image at 200dpi we would need an image file that's
4000x6000 (note again: this is the output of a 35mm drum scan).

Ultimately, this would require interpolation of most digital camera images
(including the 2560x1920 c-5050z file) to 6000x4000.

So what's the
bottom line? Can I get quality big prints from my digital camera?

Yes! (with the following caveat): it depends on which tool is used, how
much the original image is scaled and the expectations of the user.

400% enlargement seems to be an upper limit for the best tools; but I don't have
a good basis for this other than claims that I've read (yet). User
expectations also vary greatly so this number can be quite flexible.

What's the
differences between the tools?

Interpolation algorithms vary from simple methods (nearest neighbor) to complex
fractal mathematics (Genuine Fractals) to everything in between (e.g. photoshop
bicubic, lanczos, s-spline, to combinations
of multiple different types, etc.). The prices also range from free to
$160+.

The choice of tool,
depending on your preference, could become a value proposition.

Most photo editing
software will include some form of interpolation (e.g. bicubic). It's
usually found under the Image>Resize menu of the software.

Interpolation
software or a better camera?

Should you get a better camera or interpolation software?
1. what is the maximum print size you want to get of the images?
2. can interpolation software produce satisfactory results at that size?

Only you can answer
these questions.

Understanding your
quality threshold can also help determine the maximum available size for prints.